CDMA telephones operate according to the IS-95 standard. These devices operate generally as shown in the block diagram of FIG. 1. The user speaks into a microphone 100. The audio from the microphone 100 is processed by processor 102, and amplified by high power amplifier (HPA) 104. The output of HPA 104 is coupled through duplexer 108 to antenna 110. Duplexer 108 allows both transmission and reception to occur at the same time.
Transmissions received from antenna 110 are coupled through duplexer 108 to low noise amplifier (LNA) 112. The output of LNA 112 is processed by processor 114, and reproduced to the user through speaker 116.
The duplexer 110 is not perfect. It allows some transmit-to-receive crosstalk shown as 109. A measure of the transmit to receive crosstalk is available for many duplexers.
The transmitter 110 communicates with a base station shown generally as 150. Different effects occur depending on the distance between the transmitter 110 and the base station 150. When the handset 99 is close to the base station 150, the transmitter needs to transmit a relatively low amount of power and receives a relatively large amount of power. However, when the handset 99 is far away from the base station 150, it needs to transmit a large amount of power to reach the base station, and it receives only a small amount.
Other handsets which are close by are also similarly transmitting a large amount and receiving a small amount. These other handsets transceive on different frequencies. However, non-linearities in the components cause addition and different products to be formed. The other close handsets effectively become interferers.
In addition, the transmitted signal beam leaks through the duplexer as 109 and also forms another interference signal.
Interferers in non-desired bands can be eliminated by filtering, even though they may in many cases be at a higher amplitude than the actual desired signal. However, non-linearities in the LNA and/or any mixers, can cause intermodulation distortion which effectively mixes these products together. The noise from the undesired signals may be mixed into the band of desired signals. Since the noise is in the desired band, it becomes more difficult to filter out. This lowers the signal-to-noise ratio of the system as the handset gets farther from the base station.
It is also important that portable telephones have the maximum possible battery life.